The Morphology of a Solenogastre. 729 
of mollusca, of which only a few maintain a direct (cephalopods) or 
indirect (certain gastropods, pelecypods and scaphopods) possibly 
caenogenetic connection between gonad and pericardium. 
Hauer (1882) reports that a transient connection exists between 
the gonad and pericardial cavity in Chiton siculus but this is cer- 
tainly not invariably the case among the chitons. In Ischnochiton 
regularis and Isch. mertensü the gonad very early appears as a 
proliferation of cells of the anterior wall of the pericardium. In 
certain cases these are arranged in two groups while at other times 
they are continuous across the mid line but the subsequent appearance 
of two cavities in the midst of these cells on each side of the mid 
line and of the development of a median septum as in the soleno- 
gastres clearly points to the paired nature of this organ. Subse- 
quently these two cavities fuse, obliterating the septum save at the 
anterior and posterior ends where it persists for a considerable 
length of time. Concerning the development of the gonoducts I 
cannot speak positively as my work is not complete. They appear 
to be formed almost entirely as outgrowths of the coelom (gonad) 
but they are of exceedingly small size at first and require more 
careful study than I have as yet given them. 
PLare speaks of a specimen of Acanthoplewra echinata 15 mm 
long in which the gonoduct was in the form of a very slender 
ectodermic invagination which was not yet in contact with the wall 
of the gonad. It was with much surprise therefore that I found 
the gonad, containing clearly defined primordial cells, opening to 
the exterior through definite though thread-like gonoducts in several 
species of chitons some of which were not over one thirthieth of 
their adult length. In the young of Ischnochiton magdalensis, 1. 
regularis, I. mertensü, Katharina tunicata, Trachydermon raymondi and 
a number of other species none of which were more than 3 mm in 
length the gonad and its outlets had developed and bore the same 
relations as in the sexually mature animal. Although I have very 
carefully examined the above named species I have never yet seen 
the cavity of the gonad communicating with the pericardium. In 
all probability such a relation existed primitively but at the present 
day it appears to have vanished. 
Finally I would emphasize with PLATE the necessity of using 
typical members of a group in instituting comparisons and of laying 
especial stress on the broader features of their organization. Highly 
modified solenogastres do indeed exist and even in the more primitive 
